Titanium electrodes for spask plugs



TITANIUM FOR dPARK PLUGS Sven Christian Fyk and Harry Vilrtorsson, Nynashamn, Sweden No Drawing. Application March 23, 1953 Serial No. 344,2t'l2 Claims priority, application Sweden March 29, 1952 1 Claim. or. 313-141) The present invention relates to a spark plug for the ignition of a fuel-air mixture by electrical means, as in internal combustion engines.

The main parts of a spark plug of this kind consist of metal electrodes, a dielectric (insulator) and a casing which -latter forms a fixed unit together with the first-mentioned parts, the said dielectric serving to insulate at least one of the electrodes from the casing. The electric discharge which takes place between the electrodes when an adequate voltage is applied serves the purpose of igniting the fuel-air mixture.

As is known the electrodes of a spark plug undergo a change during the course of service of the plug. Under the action of the hot gases evolved during the ignition of the fuel-air mixture the electrodes are heated and become oxidized on the surface. The layer of oxide formed is punctured by subsequent ignitions, whereby the layer is broken away at some points. In consequence of this the distance between the electrodes gradually increases and is finally so great that the available voltage is no longer sufficient to puncture the layer of oxide and the gases between the electrode points, with the result that the spark plug can no longer ignite the gas mixture.

It has been found that the wear is especially marked in Z-stroke engines, which is due to the fact that in the latter ignition takes place at each revolution, whereas in a 4-stroke engine ignition only takes place in the same cylinder at every second revolution. On this account the spark plug in a 2-stroke engine is subjected to heavier thermal loading than is the case in a 4-stroke engine. Furthermore, the velocity of the gas is higher around the electrodes of the spark plug in a 2-stroke engine owing to the special shape of the combustion chamber required by the working principle of the Z-stroke engine in which scavenging of the burnt gases is effected by a new fuelair mixture. Both these factors contribute towards the attacking of the electrodes by purely chemical means.

Attempts have been made to reduce the wear on the electrodes by using specially wear resisting electrode material. For this purpose preferably nickel and nickel alloys have been used, the alloying metals being especially silicon and also chromium and copper. The nickel content in these alloys usually amounts to between 90 and 98%.

In addition, another phenomenon appears in Z-stroke engines, inasmuch as solid particles form an electrically conducting bridge between the electrodes, whereupon the normal functioning of the spark plug is rendered impossible. It is not fully understood how the formation of this bridge occurs but it is assumed to be associated with the strong eddy currents set up in the gases in the Z-stroke engine, whereby the solid particles in the gases in the combustion chamber are attracted by the influence of the electric field around the electrodes and become fastened in the electrode gap.

From the investigations on which the present invention is based, however, it has been ascertained that this bridge formation between electrodes of nickel or nickel alloys, or other ferromagnetic materials, is due in part at least to the fact that the electrodes have become magnetized, presumably by the permanent magnet of the igni- .tion system of the engine or by the action of the ignition 2,848,640 Patented Aug. 19, 1958 current itself, and the solid particles dispersed in the gases are also ferromagnetic and are thus drawn into the magnetic field between the electrodes, which contributes towards the formation of these bridges.

0n the bases of the above-mentioned observations the electrodes in the spark plug according to the invention are made of a nonmagnetizable material which at the same time possesses good mechanical strength, hardness and resistance to oxidation, whereby the drawbacks referred to are largely eliminated. In accordance therewith, according to the invention, the spark plug electrodes are made of ductile titanium.

It has been found that these electrodes absorb nitrogen, oxygen and carbon from the burnt gas mixture at temperatures which the electrodes attain during the normal functioning of the spark plug, whereby the surface hardness of the metal and therefore its mechanical resistance increase considerably without affecting the electrical conductivity of the surface, or in any case not to a detrimental extent.

As an example of the effect obtained with the spark plug electrodes of ductile titanium according to the invention it may be mentioned that comparative tests carried out in a 2-stroke engine with a fuel containing 0.4% tetraethyl lead and with electrodes made of nickel with 2% silicon gave the result that the wear amounted to 0.008 mm./ hour, whereas with the same spark plug having electrodes of ductile titanium the wear was only 0.001 nun/hour.

In another test with fuel which was free from tetraethyl lead the wear was 0.003 and 0.001 mmL/hour in the respective cases.

In this connection it may be mentioned that comparative tests have also been carried out with a tetraethylcontaining fuel, as stated above, under the same conditions with a spark plug having electrodes of titanium according to the invention and electrodes of zirconium, respectively. Although the latter metal is a non-magnetic metal with still higher resistance to chemicals and oxidizing agents than titanium, the wear of the electrodes amounted to 0.065 min/hour which is times more than in case of titanium (0.001 mrn./hour).

In view hereof, the excellent result obtained with the titanium electrodes according to the invention must be considered to be highly surprising.

What we claim is:

A spark plug consisting of metal electrodes, a dielectric insulator and a casing, all forming a fixed unit; said metal electrodes consisting essentially of non-ma-gnetizable ductile titanium having good mechanical strength, hardness and resistance to oxidation, whereby bridging between the electrodes is avoided and the electrodes during use absorb combustion gases thereby increasing their surface hardness and mechanical resistance without affecting the electrical conductivity of their surface.

References Cited in the file of this patent UNITED STATES PATENTS 352,796 Baldwin Nov. 16, 1886 896,397 Ladoff Aug. '18, 1908 1,246,054 Clough NOV. 13, 1917 1,471,326 Copland Oct. 23, 1923 2,205,854 Kroll June 25, 1940 2,629,803 Bondley Feb. 24, 1953 FOREIGN PATENTS 677,413 Great Britain Aug. 13, 1952 OTHER REFERENCES Industrial and Engineering Chemistry, February 1950, page 216.

Product Engineering, November 1949, pages 147-150. 

